External x-ray beam flattening filter

ABSTRACT

An x-ray beam flattening filter used in radiation therapy for providing uniform radiation intensity distribution across large linear accelerator produced x-ray fields, at any depth of treatment within a patient. The filter includes a solid generally planar filter member, of material semi-permeable to x-rays, and having an accurately defined surface configuration symmetrically oriented about a central axis. The filter member is affixed to a base which is designed for mounting external of the x-ray beam emergence outlet of a linear accelerator for completely intercepting the x-ray beam produced thereby, to accurately and selectively filter x-rays of the beam for producing uniform radiation intensity across the x-ray therapy fields of the accelerator apparatus.

United States Patent 1 Boge [ 1 EXTERNAL X-RAY BEAM FLATTENING FILTER [75] Inventor: Raymond J. Boge, LaCrosse, Wis.

[73} Assignee: Gundersen Clinic, Ltd., LaCrosse.

Wis.

[22] Filed: Nov. 9, 1973 [21] Appl. No: 414,363

[ 1 Nov. 4, 1975 Al/Uflte'), Agent, or Firm-Merchant & Gould 5 7 1 ABSTRACT An x-ray beam flattening filter used in radiation therapy for providing unifomi radiation intensity distribution across large linear accelerator produced x-ray fields, at any depth of treatment within a patient. The filter includes a solid generally planar filter member, of material semi-permeable to xrays, and having an accurately defined surface configuration symmetrically oriented about a central axis. The filter member is affixed to a base which is designed for mounting external of the x-ray beam emergence outlet of a linear accelerator for completely intercepting the x-ray beam produced thereby, to accurately and selectively filter x-rays of the beam for producing uniform radiation intensity across the x-ray therapy fields of the accelera tor apparatus.

1 Claim, 5 Drawing Figures 7.750 DIA.

[52] US. Cl 250/510; 250/505 [51] Int. Cl. [101.] 5/16 [58] Field 01' Search 250/510, 511,505

[56] References Cited UNITED STATES PATENTS 2,216,326 10/1940 Smith 250/510 2,405,444 8/1946 Moreau et al. 250/510 2,506,342 5/1950 Burke 250/510 2,630,536 3/1953 Vladeff 250/510 3,114,043 12/1963 Thomas et al. 250/505 3,248,547 4/1966 DeGeijn 250/510 3,678,233 7/1972 Faw et a1. 250/510 3.717.768 2/1973 Edholm et a1. 250/510 O id 3.18TDIA.

U.S. Patent Nov. 4, 1975 3,917,954

FIG.4

30x30 CM 30X 30 CM 1 4-250 DIA. 25% .0787

B, V 1 r 62b i 250" 60 EXTERNAL X-RAY BEAM FLATTENING FILTER DEFINITIONS The following definitions will be employed throughout this document:

X-Ray Beam: The projected volume of collimated xrays of polyhedron or conical shape infinitely extending from an x-ray target along a longitudinal axis.

XRay Field: That cross-sectional area of an x-ray beam defined by a plane perpendicularly intersecting the longitudinal axis of the x-ray beam and whose peripheral border is defined by the polygon or circular shaped sides of the collimated x ray beam.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to radiation beam shaping devices for use in radiation therapy and in particular to an extemally mounted x-ray filter for providing uniform radiation intensity over large x-ray treatment fields.

2. Description of the Prior Art Treatment of certain types of skin and organ diseases, most notable of which is cancer, by exposure to predetermined doses of x-rays has become commonplace in todays medical technology. In therapeutic radiology, control of the characteristics of the treating x-ray beam is all-important. Since generally high levels of radiation intensity are employed in radiation therapy, precautions must be taken to minimize exposure of and damage to those areas or tissue layers of a patient which are not directly involved in the treatment process.

Although many of the considerations and precautions in controlling an x-ray beam are identical for both diagnostic radiology and therapeutic radiology applications, the two generally differ considerably in their requirements of radiation dose distribution across any given x-ray field of the beam within a patient. In diagnostic radiology, radiographs are produced by directing the x-ray beam from an x-ray tube or port through the subject for action on and against a suitably sensitized surface. The surface is affected to a varying degree, as controlled by the attenuation of the subject to the passage of x-ray therethrough. The resulting radiograph thus varies in exposure across the x-ray field. Since the permeability to x-ray radiation of the composition of matter being radiograph varies, diagnostic radiology requires non-uniform radiation intensity distributions over specific x-ray fields for providing greater radiation intensity to certain areas of the subject being X-rayed (for example, bones, etc.), while simultaneously subjecting more xray permeable or sensitive tissues to less radiation intensity.

Radiation filters of varied construction have been employed for providing the required radiation intensity nonuniformity across an x-ray field in diagnostic radiology. These types of filters have included: filter members having custom shaped curvilinear surfaces for accomodating individual patients, wedge shaped filters positionally adjustable across the x-ray beam, multiband type filters wherein adjacent bands comprise materials having different x-ray permeability properties, and the like. Since the radiation intensity levels and time durations thereof employed in diagnostic radiology are generally low and short enough respectively so as to minimize damage by the xrays to the subject being prayed, the control problem of preciseness in uniformity of radiation dose distribution over large x-ray fields is not as acute as in the area of therapeutic radiology. Therapeutic radiology, requires the x-ray beam to define x-ray fields having uniform radiation intensity distributions across the entire area of the field at any depth of treatment within a patient. The problem is further complicated in those cases requiring treatment over large x-ray fields, for example in the treatment of Hodgkins disease. In such situations the required x-ray field of treatment may be 30cm 30cm or larger. In general, therefore. it is'important in radiation therapy to expose the treatment area to exact. known uniform radiation intensities across a treatment field at any given treatment depth with the patient. Such uniformity in radiation intensity over the entire field is necessary to prevent radiation burns of the patients skin or to prevent overdose of sensitive normal tissues within and adjacent to the treatment area during the treatment process.

In filtering the x-ray beam produced by a linear accelerator, two general techniques with respect to positioning of the filter have been employed. A first of such techniques, which to date has provided the greatest measure of success in flattening (providing uniform distribution to) the radiation intensity across a field of a collimated x-ray beam, has placed a filter within the collimating apparatus of the linear accelerator, relatively close to its x-ray target. Use of this technique, while theoretically capable of producing the desired result of flattened/uniform fields at all depths of treatment, is in practice impossible to construct, due to the extreme preciseness in shape and dimensional tolerances that would be required. A minute tolerance variation in any dimension of such a filter projects significant and undesired radiation intensity differences to the remotely positioned x-ray treatment field.

The second general technique for filtering therapeutic x-rays requires placement of the filter between the x-ray beam outlet of the linear accelerator and the field of treatment. One such filter used in radiation treatment of tumors employs a layer of metal having a grid pattern therein and placed in the path of the x-ray beam for absorbing a portion of the soft rays of the beam which would damage the skin tissue while permitting the hard rays of the beam to pass therethrough to the treatment area. This type of filtering technique, however, does not compensate for any basic nonuniformity in the radiation intensity of x-ray beams across the various fields of treatment. Another approach provides a set of standardized compensating filters for mantle-field therapy, which is intended to provide non-uniform field intensities at various treatment depths within the patient, and is similar in function to the diagnostic radiology filters previously described.

The beam flattening filter of my invention overcomes the practical inaccuracies and inadequacies of the prior art beam flattening filters, by simultaneously providing uniform radiation intensity over entire large area x-ray fields of an x ray beam, at any practical treatment depth. While my invention will be described in conjunction with its use in a specific linear accelerator apparatus, it will be understood that it is not limited to this use, but can be employed in any linear accelerator x-ray producing apparatus. Further, while the present invention is described herein as particularly applicable 3 to use is radiation therapy. it will be understood that my invention is not limited to this use but could well be employed in diagnostic radiology and other applications requiring a uniform radiation intensity distribution across a x-ray field. It will further be understood that the specific geometrical surface configurations (including dimensions and angles) of the preferred embodiment beam flattening filter described herein apply directly to use with the specific linear accelerator described, and that such angles, dimensions and geometric relationships could be appropriately modified within the spirit and intent of my invention to apply to other accelerator apparatus.

SUMMARY OF THE INVENTION The x-ray beam flattening filter of this invention comprises a solid, generally planar filter member semipermeable to x-rays and peripherally shaped to the cross-sectional configuration (field) of the x-ray beam to be filtered. At least one of the planar-like surfaces of the filter member is precisely geometrically configured to provide predetermined varied attenuation across the x-ray field to x-rays passing therethrough. The filter member is designed for placement within an x-ray beam which is generally symmetrically collimated about a longitudinal axis such that the longitudinal beam axis is perpendicular to the general plane of the filter member and passes through a geometrical center of the configured surface of the filter member. The filter member is designed for intercepting the x-ray beam intermediate the collimating apparatus of the linear ac celerator and the treatment fields for a patient, to provide uniform radiation intensity completely across the x-ray treatment fields at all depths of treatment.

The filter member may be mounted upon a shadow tray, mounted externally of the linear accelerator head. or may be accurately affixed to a mounting base of material having uniform x-ray permeability thereacross for mounting on a bracket adjacent the x-ray emergence outlet of the linear accelerator.

BRIEF DESCRIPTION OF THE DRAWINGS Referring to the Figures wherein like numerals and letters represent like parts throughout the several views;

FIG. 1 is a diagrammatic representation of a typical linear accelerator x-ray producing and collimating apparatus employing the beam flattening filter of the present invention;

FIG. 2 is a top plan view of a preferred embodiment of the beam flattening filter of this invention, disclosed in FIG. 1;

FIG. 3 is a cross-sectional view of the beam flattening filter disclosed in FIG. 2, generally taken along the line 3-3 of FIG. 2;

F IG. 4 is a graph of typical isodose distribution curves referenced to the radiation intensity at the dose maximum of an of a megavoltage x-ray beam as they would typically appear for a linear accelerator apparatus of the type disclosed in FIG. 1, without the benefit of the beam flattening filter of this invention; and

FIG. 5 is a graph of the typical isodose distribution curves of FIG. 4 as they would appear when flab tened" by the beam flattening filter of this invention as disclosed in FIGS. 2 and 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. I, a linear accelerator is generally diagrammatically illustrated at 20. That portion of the linear accelerator which produces x-rays comprises: an electron gun 21 for projecting electrons therefrom. an electron accelerator section 22 having an RF input 23, and an x-ray target 24. The principles of x-ray production by linear acceleration apparatus are well known in the art, and will not be belabored herein. In general, however, the accelerator section 22 directs and accelerates electrons produced by the electron gun 2] toward the xray target 24 at high velocities. In the preferred embodiment, the x-ray target 24 is comprised of tungston. In the process of deceleration, the accelerated electrons bombarding the x-ray target 24 give up energy in the form of x-rays which are directed toward a primary collimator 25. The primary collimator 25 is supported by the support assembly generally designated at 26. The primary collimator 25 generally comprises a mass of depleted uranium having a truncated conical hole 27 symmetrically formed therein about a longitudinal axis 30 which extends through the center of the x-ray target 24.

An ion chamber having a primary filter 36 connected thereto are operatively connected in spaced relationship with the primary collimator 25 such that the primary filter 36 is symmetrically aligned about the longitudinal axis 30. In the preferred embodiment, the primary filter 36 has a symmetrical spherically or hyperbolically shaped surface directed into the oncoming path of the x-rays.

A pair of upper collimating jaws 40 (one of which is illustrated in FIG. 1) are symmetrically opposed about the longitudinal axis 30 for providing secondary collimation of the x-rays. The upper collimating jaws 40 are operatively connected (not illustrated) for movement toward and away from the longitudinal axis 30 in a direction normal to the plane of the paper when viewed as in FIG. 1. A pair of lower collimating jaws 42 are symmetrically disposed about the longitudinal axis 30 for providing further collimation of the x-rays. The lower collimating jaws 42 are operatively connected by apparatus (not illustrated) for movement in the plane of the paper as viewed in FIG. 1 at right angles to the longitudinal axis 30. The upper and lower collimating jaws 40 and 42 respectively, are typically comprised of depleted uranium and define the outer periphery of a beam of the x-rays, projected downwardly therefrom and generally designated by the number in FIG. 1.

A housing cover 51 encloses the ion chamber 35, the primary filter 36, and the collimating jaws 40 and 42 and the movement and alignment apparatus (not illustrated) associated therewith. An opening 52 in the bottom portion of the housing 51 defines an emergence outlet therefrom for the x'ray beam 50. That portion of the linear accelerator apparatus, sequentially including the electron gun 21 through the emergence outlet 52, is often referred to as the head of the linear accelerator.

A pair of mounting brackets 55 are connected to the housing 51 adjacent the outlet 52 for holding accessories. In FIG. 1, the brackets 55 are illustrated as holding a first shadow tray 56 and an external beam flattening filter apparatus, generally designated at 58, comprising my inventlon.

The linear accelerator apparatus illustrated in FIG. 1 and that for which the external beam flattening filter apparatus 58 of the preferred embodiment was specifically designed, is the Clinac 4 manufactured by \"arian Associates (Radiation Division) and de scribed in their brochure Rad 1568C 3M 4-69". The primary filter 36 employed within the Clinac 4 was described in a paper by Henning Hensen at the annual meeting in Houston, Texas, in 1971 of the American Association of Physicists in Medicine.

A more detailed illustration of the external beam flattening filter apparatus 58 is illustrated in FIGS. 2 and 3. Referring to FIGS. 2 and 3, there is generally illustrated a mounting base 60, of plexiglass material in the preferred embodiment. sized for accurate alignment and positioning within the mounting brackets 55 of the linear accelerator 20. The mounting base is of sufficient thickness and rigidity to prevent bending under its own weight when held by its edges in the mounting brackets 55. In the preferred embodiment, the filter member 62 is comprised of brass. However, other solid materials having known x-ray permeability properties may be employed within the spirit and intent of this invention.

A filter member, generally designated at 62, is accurately positioned and affixed to the base 60. In the pre ferred embodiment, the filter member 62 is generally disc shaped and is symmetrical about a central axis 64 extending perpendicular to the general plane of the filter member 62. The filter member 62 is accurately positioned upon the mounting base 60, such that the central axis 64 will be aligned colinear with the longitudinal axis 30 of the x-ray beam 50 when the base 60 is 0peratively secured by the mounting brackets 55 of the linear accelerator 20.

The filter member 62 generally has a planar lower surface 62a directly attached to and forming an inter face with the upper surface of the base member 60, and a geometrically precisely configured upper surface 62b. The upper surface 62b is symmetrically configured about the central axis 64, and in the preferred embodiment is configured to precisely define a thickness of brass between the upper and lower surfaces 62b and 62a for providing uniform radiation intensity across an entire field of the x-ray beam 50, for the Clinac 4 type linear accelerator.

In the description to follow, it will be understood that although the filter member 62 will be described in terms of a plurality of segments or portions thereof, this manner of description is for the purposes of defining the particular unique goemetrical configuration of the filter members upper surface 62b, and that the entire filter member 62 comprises a single integral unit.

The upper geometrically configured surface 62b of the filter member 62 generally comprises a first flat ring-like surface 65, defining a washer shaped volume of uniform cross-sectional thickness relative the lower surface 62a and symmetrically disposed about the central axis 64. A second downwardly sloping surface 66 radially extends from the externally directed upper peripheral edge of the ring-like portion 65 of the filter member 62 to the interface 62a with the mounting base 60. The second surface 66 defines a second volume of triangular cross-sectional area relative the lower surface 62a. The external radially directed edge of the second surface 66 of the filter member 62 also defines the external periphery of the filter member. In the preferred embodiment, the radially slanting peripherial surface 66 is formed at an angle of degrees 15 min- 6 utes in the radial direction with respect to the lower surface 620 of the filter member 62.

A third distinct segment of the upper surface 621) of the filter member 62, generally designated at 67. comprises a downwardly sloping surface radially extending from the internal peripheral edge of the upper surface of the ring-shaped portion 65 of the filter member, and forms an angle of 1 degree 49 minutes therewith. The third surface 67 downwardly slopes in the direction of the central axis 64 and defines, with the lower surface 62a of the filter member 62, a symmetrical volume thereabout having a trapezoidal cross-sectional area.

A fourth distinct surface area 68 of the upper surface 62b of the filter member 62 is contiguous with the internally directed edge of the third surface 67 thereof, and extends radially inward therefrom toward the cen tral axis 64. The fourth surface slopes downwardly toward the central axis 64 forming an angle of 2 degrees 4 minutes with the plane of the first surface 65 of the filter member 62. The fourth surface 68 defines with the lower surface 62a, a volume of triangular cross-sectional area symmetrically disposed about the central axis 64, whose internally directed peripheral edge defines a hole 69, of the filter member 62. The hole 69 is axially aligned with the central axis 64.

The dimensions for that preferred embodiment of the beam flattening filter apparatus 58 which have been formed to be specifically adapted for use with the Clinac 4 linear accelerator are detailed in FIGS. 2 and 3.

A graph of the typical isodose distribution curves within the treatment area for that type of linear accelerator 20 illustrated in FIG. 1 which employs the primary filter 36, is illustrated in FIG. 4. The use of isodose curves is conventionally employed in the art to define radiation intensity distributions, and will not be detailed herein. The isodose curves represent the convention of defining 100% of dose on the longitudinal central axis of the x-ray field and at a point on the axis spaced a known distance from a phantom surface 72, and called dose maximum". In FIG. 4, the dose maximum is designated at 76.

In the preferred embodiment, the dose maximum 76 is located on the longitudinal central axis 30 a distance of 8L2 cm from the x-ray target 24. All further percentages of dose are referenced to the l00% of dose position (i.e., to dose maximum 76). Therefore. that point designated at 5 cm in FIG. 4 on the axis 30, is in reality cm from the x-ray target 24, and 5 cm below the phantom surface 72.

Several of the x-ray fields used in treatment of a patient are represented by horizontal dashed lines at 77 in the graph of FIG. 4. It will be understood that an infinite number of such fields exist within the treatment area. The peripheral edge of each isodose curve is defined by the x-ray beam edge 50, and has associated therewith a penumbra, generally designated at 75, representing the typical shadowing effect of incomplete collimation of the x-ray beam 50. It will be understood that the isodose curves illustrated in FIGS. 4 and 5 represent the variation in radiation intensity distribution across the x-ray beam 50 at those various distances (in cm) from the dose maximum 76, and that the isodose curves are merely a two-dimensional representation of the radiation intensity distribution across the three-dimensional x-ray beam.

It will also be understood that the size of the x-ray field (designated as 30 cm X 30 cm in FIG. 4) is that cross-sectional area of the x-ray beam 50 as measured at the x-ray field passing through the phantom surface 72. The size (cross-sectional dimensions) of those x-ray fields 77 placed downstream in the x-ray beam 50 from the dose maximum 76 will necessarily increase in size, according to the inverse square law, with respect to their axial distance from the x-ray target 24.

In radiation therapy, it is highly desirable to have uniform radiation intensity distribution across an entire x-ray field at any depth of treatment. Without the primary filter 36 intercepting the x-ray beam 50, the isodose curves would be highly non-uniform across any field 77 of the beam within the treatment area. The primary filter 36 provides some degree of uniformity in radiation intensity distribution across the fields 77 in the treatment area. However, it does not provide that degree of uniformity (flatness) required across large x-ray fields in the treatment area. Referring to FIG. 4, it will be noted that while the top 100%) isodose curve has a radiation intensity of IOO% of dose at the position of dose maximum 76 on the longitudinal central axis 30, the radiation intensity near the edges of the beam 50 of an xray field passing through the dose maximum 76 is approximately I I7% of dose. Therefore, any tissue exposed to the higher radiation at these outer edges of the field could be subjected to radiation burns. A similar non-unifonnity in the radiation intensity distribution across the x-ray fields, at all treatment depths will be noted from FIG. 4. Besides endangering surface burns of skin and subcutaneous tissue of a patient being treated, such an isodose curve distribution causes nonuniform radiation intensity to be applied to the specific tissues being treated at the lower treatment depths. Submaxillary nodes, salivary glands, the thyroid gland and stemal bone marrow could receive excessive radia tion doses expecially if the radiotherapist were accustomed to think in temts of I% of dose occurring on the longitudinal central axis of the large field.

FIG. is a graph of the same isodose curves for that type of linear accelerator illustrated in FIG. 1, with the beam flattening filter apparatus 58 of this invention employed therewith. The precisely contoured upper surface 62b of the filter member 62 of the beam flattening apparatus 58 removes that non-uniformity of radiation intensity across the x-ray treatment fields 77 otherwise present at all depths of treatment, as previously illustrated in FIG. 4.

Referring collectively to FIGS. 2, 3 and 5, and assuming that the beam flattening filter apparatus 58 of this invention has been properly mounted in the path of the x-ray beam external of the emergence outlet 52, it will be noted that the hole 69 at the center of the filter member 62 allows unimpeded passage of x-rays therethrough along and adjacent the longitudinal axis 30. Therefore, the 100% of dose radiation intensity characteristic of the linear accelerator 20 will remain at the same dose maximum position as previously described. The fourth and third surfaces 68 and 67 respectively of the upper surface 62b of the filter member 62, are respectively sloped at predetermined angles to compensate for that portion of increasing radiation intensity across the treatment fields 77, generally designated at 80a in FIG. 4. The first washer-shaped portion 65 of the filter member 62 is of uniform cross-sectional thickness to provide uniform attenuation to the x-ray beam at that portion thereof generally designated at 80b in FIG. 4. The second portion 66 of the filter member 62 is shaped to compensate for the rapid decrease in radiation intensity near the peripheral edges of the x-ray beam, generally designated at c in FIG. 4. The result of placing the beam flattening filter apparatus 58 of the preferred embodiment so as to intercept the x-ray beam 50 of a Clinac 4 linear accelerator 20, is that the radiation intensity distribution across the entire beam is flattened to provide uniform radiation intensity across an entire x-ray field at any depth of treatment as illustrated by the isodose curves of FIG. 5. In the preferred embodiment, the beam flattening filter apparatus 58 is spaced approximately 50 cm. from the x-ray target 24. The beam flattening filter apparatus of the preferred embodiment, above described, produces less than 3% radiation intensity variation over a 30 cm X 30 cm field at the position of dose maximum.

While I have disclosed a specific embodiment of my invention, it will be understood that this is for the purpose of illustration only, and that my invention is to be limited soley by the scope of the appended claims.

What is claimed is:

1. An external filter for flattening a plurality of large x-ray fields of an x-ray beam produced by a linear accelerator of the type having means for producing xrays, means for collimating the x-rays into an x-ray beam having a longitudinal axis, and first filter means in said collimating means for roughly shaping said fields of the beam, said external filter comprising:

A. a single solid disc-shaped member semi-permeable to said x-rays and peripherally symmetrically shaped in the configuration of said x-ray field about a central axis, said disc-shaped member comprismg:

i. a planar lower surface lying in a plane perpendicular to the central axis; and

ii. a geometrically shaped upper surface defined relative said lower surface by:

a. a first ring-shaped surface parallel to said lower surface, defining a washer shaped volume therebetween about said central axis;

b. a second surface linearly radially extending from said lower surface to an outwardly directed peripheral edge of said first surface, defining a ring having a geometrical volume of triangular cross-sectional area about said central axis;

c. a third surface radially extending from an in ternally directed edge of said first surface toward said central axis and terminating at a circle thereabout, defining a ring having a geometrical volume of polygon shaped cross-see tional area about said central axis; and

d. a fourth surface forming a radially directed ex tension of said third surface in the direction of said central axis and terminating at said lower surface, defining a ring having a geometrical volume of triangular cross-sectional area about said central axis; and

B. mounting means of x-ray permeable material, having a planar mounting surface to receive the lower surface of said disc-shaped member for mounting said disc-shaped member thereon, said mounting means being adapted for mounting alignment with said linear accelerator such that said longitudinal beam axis and said disc central axis are colinear. 

1. An external filter for flattening a plurality of large x-ray fields of an x-ray beam produced by a linear accelerator of the type having means for producing x-rays, means for collimating the x-rays into an x-ray beam having a longitudinal axis, and first filter means in said collimating means for roughly shaping said fields of the beam, said external filter comprising: A. a single solid disc-shaped member semi-permeable to said xrays and peripherally symmetrically shaped in the configuration of said x-ray field about a central axis, said disc-shaped member comprising: i. a planar lower surface lying in a plane perpendicular to the central axis; and ii. a geometrically shaped upper surface defined relative said lower surface by: a. a first ring-shaped surface parallel to said lower surface, defining a washer shaped volume therebetween about said central axis; b. a second surface linearly radially extending from said lower surfaCe to an outwardly directed peripheral edge of said first surface, defining a ring having a geometrical volume of triangular cross-sectional area about said central axis; c. a third surface radially extending from an internally directed edge of said first surface toward said central axis and terminating at a circle thereabout, defining a ring having a geometrical volume of polygon shaped cross-sectional area about said central axis; and d. a fourth surface forming a radially directed extension of said third surface in the direction of said central axis and terminating at said lower surface, defining a ring having a geometrical volume of triangular cross-sectional area about said central axis; and B. mounting means of x-ray permeable material, having a planar mounting surface to receive the lower surface of said discshaped member for mounting said disc-shaped member thereon, said mounting means being adapted for mounting alignment with said linear accelerator such that said longitudinal beam axis and said disc central axis are colinear. 